Analyzers using the so-called dried test elements in which the reagents are pre-incorporated in a dried format, have allowed a large number of different tests to be conducted. These include slide-like test elements that react colorimetrically to assay, for example, for phosphorous and CKMB. In the past, phosphorous tests have been incubated in end-point incubators, along with other end-point assays, while CKMB has been incubated in a separate rate-assay incubator.
To simplify the analyzer, there has been a need to make one incubator, hereinafter "single incubator", function for both rate and end-point assays. However, there has been a problem prior to this invention when phosphorous test elements have been incubated together with CKMB test elements. For reasons which were not clear, if phosphorous had been tested at an incubator station prior to a CKMB test element, the latter produced a higher reading than it should have. Although the operator could endeavor to run CKMB tests at a totally different time than phosphorous tests, this is an unacceptable limitation for most laboratories, particularly those involved in STAT operations, that is, those that must be run immediately.
Yet another problem encountered by certain test elements is that their detected levels of analytes have been routinely higher when tested on this single incubator, than expected, particularly when compared to the reading obtained on a dedicated colorimetric analyzer. These are elements in which the spreading layer first contacted by the sample is a beaded spreading layer of the type described in U.S. Pat. No. 4,258,001, particularly useful in assays such as total protein and albumin. Again, the cause of this positive bias has not been clear prior to this invention. It will be understood that such bias could be acceptable if the math modeling were adjusted to correct for the bias (when providing the conversion of reflectance to concentration.) However, in switching to a single incubator for both rate and colorimetric assays, it is preferred that as much of the software used in the conventional colorimetric analyzers and rate analyzers, be adapted or transferred to the single incubator, as possible. Since the bias present, e.g., in total protein takes the values completely out of the "normal" reading domain heretofore used, such transfer heretofore has not been possible.
Japanese Kokai No. 63/47661 published Feb. 29, 1988 teaches the use of a wide variety of non-porous, non-absorbing polymers for evaporation covers at incubator stations featuring dried test elements. The polymers listed as being useful are polyolefins such as polyethylene and polypropylene, polyesters such as polyethylene terephthalate, butylene terephthalate and polyethylene-polybutylene terephthalate. The purpose is stated to be to reduce the absorption of any reactive gas produced by a test element. However, the only reactive gas that is specifically discussed is ammonia from BUN test elements. There is no appreciation of the cause of the problem created when one incubator has to be used for both phosphorous test elements and CKMB test elements. There is, furthermore, no discussion of a colorant to be used in the cover, or of problems associated with test elements constructed using beaded spreading layers.